The present invention relates, in general, to optical parametric generators having high repetition rate, femtosecond output pulses tunable over a large range in the near infrared, and more particularly to tunable optical parametric oscillators (OPO) and optical parametric amplifiers (OPA) utilizing nonlinear crystals. The invention further relates to tunable, noncritically phasematched optical parametric generators utilizing nonlinear crystals capable of operation in any x-y crystal orientation.
Laser pulses in the femtosecond time domain have become important in the study of extremely short-duration events such as chemical reactions, and for studying the dynamics of high-speed semiconductor electronic and optical devices. A particularly interesting region of optical study lies in the near to mid-infrared, and for this purpose a tunable source of optical pulses in that wavelength range is required.
Femtosecond OPOs have been demonstrated which exploit the high peak powers and high-repetition rates of the femtosecond Ti:Sapphire laser to produce high-repetition-rate femtosecond pulses broadly tunable in the near and mid-infrared. In particular, the nonlinear crystal KTiOPO.sub.4 (KTP) has been used successfully in a critically phase-matched geometry in both internally and externally-pumped OPO configurations in the femtosecond time domain. See, for example, Edelstein et al, "Broadly Tunable High-Repetition Rate Femtosecond Optical Parametric Oscillator", Physics Letters, Volume 54, page 1728 (May 1, 1989); Wachman et al, "Continuous-Wave Mode-Locked and Dispersion-Compensated Femtosecond Optical Parametric Oscillator", Optics Letters, Volume 15, Page 136 (1990); and Wachman et al, "CW Femtosecond Pulses Tunable in the Near- and Mid-Infrared", Journal of Applied Physics, Volume 70, Page 1893 (1991), as well as U.S. Pat. No. 5,017,806 to Edelstein et al, May 21, 1991. See also U.S. Pat. No. 5,377,043, issued Dec. 27, 1994, to Pelouch et al, entitled "Ti:Sapphire-Pumped, High Repetition Rate Femtosecond Optical Parametric Oscillator", and assigned to the assignee of the present application, the disclosure of which is hereby incorporated herein.
In the latter application, high power, high repetition rate, femtosecond pulses are produced over a broadly tunable range through the use of an externally pumped, self-mode-locked Ti:Sapphire laser. The output of the Ti:Sapphire laser is focused onto one surface of a nonlinear crystal, such as a KTP crystal, located in an optical parametric oscillator cavity. Tuning of the optical parametric oscillator is accomplished by three different mechanisms in that device. A first technique involves adjusting the length mismatch of the OPO cavity with respect to the pump cavity to produce a wavelength shift, with a wavelength range of 50 nm being obtained for a variation of 5 .mu.m in the length of the cavity. A second way to tune the output of the OPO is to change the pump wavelength without changing the orientation of the crystal or the alignment of the OPO. A third way is to use the traditional method of adjusting the phase-matching angle of the crystal so that by rotating the crystal and adjusting the cavity length, the output of the OPO can be tuned over a wide range. However, there is still a need to simplify the operation and extend the wavelength range of the optical parametric oscillator.